Reduced sulfur gases are present in many industrial processes. For example, reduced sulfur gases are found in flue gas, coal gas and fuel gas streams. They are also found in industrial product gas streams such as olefin-containing gas streams, which are a component of petroleum refining operations. These gases are often removed from such gas streams by use of various metal oxides that have the ability to capture a reduced sulfur-containing gas component from such gas streams. In order to capture such a gas from certain industrial processes (such as packed-bed, fluidized-bed or moving-bed reactors), the metal oxide, reduced sulfur gas sorbent materials must be used in forms that are mechanically strong and resistant to attrition. Otherwise, problems, such as pressure drops through a process reactor unit, particulate matter elutriation and/or clogging of valves or other mechanical components will take place.
Moreover, almost all industrial processes that deal with reduced sulfur gases also are confronted with the problem of desorbing these gases from the metal oxide sorbent material so that the sorbent material can be used over and over again in order to obtain its maximum economic benefit. Other problems associated with the presence of reduced sulfur gases (such as H2S, COS and CS2) in gas streams such as fuel gases, flue gases and waste gases arise from the fact that reduced sulfur gases are corrosive toward ferrous metals. They are especially corrosive toward steel turbine blades. Therefore, the presence of reduced sulfur gases in those hot fuel gases used to power turbines results in their severe corrosion. Oxidation of hot fuel gases also serves to oxidize any reduced sulfur gases contained therein. The resulting sulfur oxide gases (e.g., SO2, and SO3, which are commonly referred to as “SOx” gases) also are highly corrosive toward ferrous metals. Moreover, upon release to the atmosphere, SOx gases form so-called “acid-rain.” Therefore, the concentration of reduced sulfur gases contained in those hot fuel gases introduced into power-producing equipment, such as turbines and fuel cells, must be brought to very low concentrations, e.g., a few parts per million (ppm), before they are combusted in equipment of this kind.
Next, it should be noted that in the case of sulfur oxide sorbents—as opposed to the reduced sulfur sorbents that form the subject matter of this patent disclosure—the subject sulfur oxide is normally usually captured in an oxidizing atmosphere such as those extant in the catalyst regenerator of a FCC unit. This is done through use of various metal oxide particles having an affinity for a given sulfur oxide-containing gas. These particles are often entrained in a “fluidized” process. For example, sulfur oxide (e.g., SO2 or SO3) sorption is often carried out through the use of fluidized microspheroidal magnesium-containing particles that can withstand the hot (e.g., 1350° F.) oxidizing conditions present in the catalyst regenerator units of those fluid catalytic conversion (“FCC”) processes used to refine petroleum. Conversely, release of such sorbed sulfur-containing gases usually occurs in the reducing environment of a FCC reactor. The sulfur component of such gases is released as hydrogen sulfide H2S. This released H2S gas is readily captured downstream of the reactor and normally does not create an environmental hazard.
Again, however, the processes of the present patent disclosure are different from such SOx sorption processes in that applicant's compositions are specifically designed to capture chemically reduced forms of sulfur (e.g., those in H2S, COS and CS2) rather than chemically oxidized forms of sulfur (e.g., those in SO2 and SO3). Thus, applicant's capture of reduced sulfur gases must take place under chemical reduction conditions, rather than under chemical oxidizing conditions.
Many different zinc-containing compounds have been used in both fixed bed and fluid bed systems in order to remove one or more species of reduced sulfur gases from various industrial gas streams (e.g., fuel gases, such as those derived from the gasification of coal, flue or waste gases and/or industrial product gases, such as those that contaminate olefin-type gases). Such zinc-containing compounds have included zinc oxide, zinc titanate and zinc aluminate. Zinc oxide, for example, has been used as a sorbent for selectively removing hydrogen sulfide gas, H2S, from certain industrial gas streams. This metal oxide is normally used by placing it in contact with a hydrogen sulfide-containing gas stream at elevated temperatures. Zinc oxide, in and of itself, has not, however, proven to be a particularly effective hydrogen sulfide sorbent for many industrial applications. For example, its hydrogen sulfide sorption ability is relatively limited, especially at lower temperatures. It also suffers from the drawback of not being easily regenerated. This drawback follows from the relatively high thermodynamic stability of the zinc sulfide product of zinc oxide-hydrogen sulfide reactions. Zinc oxide also lacks the qualities of hardness, toughness and/or attrition resistance that are needed for many industrial applications.
Regeneration of the zinc sulfide product of zinc oxide-hydrogen sulfide reactions requires subsequent oxidation of the sulfur component of the zinc sulfide reaction product. This must be done at relatively high temperatures (e.g., 900° F. to 1500° F.). Unfortunately, the relatively high temperatures needed to oxidize zinc sulfide back to zinc oxide also tend to degrade the already inherently low mechanical strength and/or toughness of these zinc oxide-based materials. Consequently, zinc oxide sorbents tend to quickly disintegrate when they are repeatedly used and regenerated.
Therefore, in order for zinc oxide-containing compounds to be effectively used in the harsh environments where they are needed (e.g., in the high temperature/high velocity particle impact environments of fluid, fixed or bubbling bed processes), they must be combined with other tougher and more attrition resistant metal oxide components in order to produce overall zinc oxide/metal oxide compositions having the requisite mechanical strength, hardness, durability, toughness and attrition resistance that they will need to function as reduced sulfur gas sorbents.
Generally speaking, this has been accomplished by mixing certain prescribed proportions of a relatively soft zinc oxide component with certain prescribed portions of another, relatively harder, tougher, metal oxide component in the same particle. The most effective and widely used metal oxide used for this hardening/toughening purpose has been unreacted alumina (Al2O3). Such use of alumina as a catalyst support for zinc oxide sorbents follows from the unusually high degree of hardness this material imparts to such compositions, as well as from the excellent binding capabilities of many forms of so-called “gelling” or “sol” alumina forms. Examples of such aluminas are the various grades of VISTA CATAPAL® and CONDEA DISPERAL® aluminas. Such aluminas have been used in producing relatively harder, tougher and more attrition resistant extrudate, granule, microsphere, powder, particle, pellet, bead, etc. forms of zinc oxide/unreacted alumina compositions.
Improvements in the sorption, regeneration and physical attributes of zinc oxide-containing compositions are described, for example, in U.S. Pat. No. 4,088,736 (“the '736 patent”) which discloses a reduced sulfur sorbent comprised of homogeneous mixtures of zinc oxide, alumina, silica, and Group II-A metal oxide(s). The alumina and silica components of the compositions taught by the '736 patent serve to impart toughness and attrition resistance to the therein disclosed compositions.
Other patent disclosures teach the use of other zinc-containing compounds, i.e., other than zinc oxide, as the “active,” reduced sulfur gas-capturing agent in such compositions. For example, U.S. Pat. No. 4,263,020 (“the '020 patent”) discloses the reduced sulfur gas-capturing abilities of metal aluminate spines having the general formula MAl2O4 wherein the M component can be chromium, iron, cobalt, nickel, copper, cadmium or mercury. Zinc aluminate spinel, ZnAl2O4, is a particularly preferred member of this group of compounds. The '020 patent also discloses that the zinc atoms of such a zinc aluminate spinel form simple adsorption bonds with reduced sulfur gases. These adsorption bonds are sufficient to remove a reduced sulfur gas, such as hydrogen sulfide, from a recycle hydrogen gas stream. The '020 patent also discloses that, unlike the chemical mechanism involved in the removal of reduced sulfur gas (e.g., hydrogen sulfide) from a recycle hydrogen gas stream by the use of zinc oxide-based sorbents, there is no chemical reaction in the process of the '020 patent wherein zinc sulfide is ever formed from these MAl2O4 compounds. Consequently, they can be regenerated by simply purging or sweeping the physically sorbed, reduced sulfur gas from these MAl2O4 compounds with a hot, inert gas such as nitrogen.
Zinc titanate has also been used as a reduced sulfur compound sorbent. Indeed, it has been used in sorbents having no binder component other than the zinc titanate itself. Unfortunately, zinc titanate (like zinc oxide) also suffers to some degree from the drawback of being a relatively “soft” material. Consequently, most zinc titanate-containing compositions (like zinc oxide-containing compositions) employ an unreacted alumina component as a binder material in order to create hard, tough, attrition resistant zinc titanate/unreacted alumina compositions.
Still other zinc titanate-containing, reduced sulfur sorbent compositions do not employ unreacted alumina, but rather employ other kinds of toughness-imparting binder materials. For instance, U.S. Pat. No. 5,254,516 (“the '516 patent”) teaches various zinc titanate-based sorbent materials that further comprise a combination of inorganic and organic materials that are used as a binder for the active zinc titanate ingredient. These inorganic binder materials include clays, such as kaolinite and bentonite (which are aluminosilicates), feldspar, sodium silicate, forsterite and calcium sulfate. The more preferred organic binders used to create the tough binder materials disclosed in the '516 patent are methylcellulose-based compositions such as that commercially available as METHOCEL®.
Those skilled in this art also will appreciate that zinc titanate-containing compositions tend to lose more and more of their capacity to absorb reduced sulfur gases as more and more of the binder ingredients (e.g., clay, alumina, organics, etc.) are used in these compositions. It might also be noted in passing here that it was heretofore generally believed that the reason for this loss in reduced sulfur gas capturing ability was simply due to the fact that less zinc titanate is present in those compositions having relatively greater proportions of binder materials.
The present invention addresses a need in the art for new and improved materials with sulfur gas capturing capacity and provides such materials and methods of production and use thereof.